In driving of heavy vehicles such as trucks, buses and the like, vehicle economy has over time become an ever greater factor in the profitability of the activity in which the vehicle is used. Apart from its procurement cost, the main costs involved in routine operation of a heavy vehicle normally comprise pay for the driver, repair and maintenance costs, and fuel for propulsion of the vehicle.
Depending on the type of vehicle, the impact of different factors may vary, but fuel consumption is generally a major item of expenditure, and since the capacity utilisation of heavy vehicles is often high, involving a large overall fuel consumption, every possible way of reducing fuel consumption has a positive effect on profitability.
On long-distance runs it is particularly important to optimise fuel consumption. To this end there are long-distance vehicles characterised by a typical engine cruising speed for a certain vehicle cruising speed. Typical vehicle cruising speeds, depending on the region or type of road, may for example be 80 km/h, 85 km/h or 89 km/h.
For heavy vehicles in general, various different power train configurations are available. But, since it is often desirable that such vehicles be drivable as comfortably as possible for the driver, they are often provided with automatically operated gearboxes such that gear changes are controlled by the control system usually incorporated in the vehicle.
The fact that automatic gear change systems in heavy vehicles are usually controlled by the control system makes it possible, a possibility which is often applied, to use a control arrangement whereby the control of the engine and gearbox is conducted partly on the basis of commands from the vehicle driver but also to a large extent by the control system. For this reason the control system often also incorporates functions for improving fuel consumption by as far as possible effecting gear changes and gear choices in as fuel-economising a way as possible.
An example of such a function is a function whereby when the vehicle is on a downgrade, its engine is disconnected from its powered wheels when no torque contribution is required to maintain the vehicle's speed. The vehicle's power train is subsequently reconnected when, for example, the driver presses the accelerator pedal or the brake pedal.
Although the aforesaid disconnecting function may work well in many cases, there are still situations where the fuel consumption of vehicles powered by a combustion engine can be further reduced.